Linear polyesters typically are prepared from oligomers made by reacting together one or more dicarboxylic acids and one or more diols via direct esterification, by reacting together one or more dimethyl esters and one or more diols via transesterification, or by carrying out both direct esterification and transesterification in a single reaction mixture. Water evolves from the reaction mixture in the case of direct esterification, and methanol evolves from the reaction mixture in the case of transesterification. The resulting oligomers may be converted to higher molecular weight polyester polymers via polycondensation. Branched polyesters may be made by introducing tri- or higher-functional reactants in place of some of the dicarboxylic acids, diols or dimethyl esters.
Low molecular weight polyesters normally are prepared in a single stage reaction that accomplishes both direct esterification and polycondensation. The reaction typically is carried out at atmospheric pressure and at temperatures near the normal boiling point for the diol (e.g., at temperatures of about 170-210° C. for reactions using ethylene glycol). A large diol excess normally is employed. A small quantity (e.g., about 3%) of xylenes may be added near the end of the reaction to assist in distilling water from the reaction mixture. The end product is a low molecular weight polyester which after cooling to room temperature may be a liquid or in some cases an amorphous solid.
Medium and high molecular weight polyesters typically are made via a two stage process. The first stage typically is a direct esterification or transesterification reaction to form a liquid low molecular weight oligomer and the second stage typically is a polycondensation reaction to convert the oligomer to a polymer with a targeted molecular weight. Considerable time may be required to complete the two stages. The first stage esterification reaction may for example be carried out using conditions similar to the low molecular weight polyester direct esterification reaction conditions described above. The second stage polycondensation reaction typically is performed using melt or solid state polymerization, together with vacuum (e.g., about 0.1-1 mm pressure) and high temperature (e.g., temperatures above ambient temperature such as about 270-290° C. for polyesters derived from ethylene glycol). The vacuum and heat aid in removal of the excess diol. The reaction mixture typically has sufficiently high viscosity so that it would be unduly difficult to stir it during the polycondensation reaction. The end product is a medium or high molecular weight polyester which after cooling is a solid. The solid product typically is pelletized prior to shipment to an end user. The end user may in turn melt the pellets using an extruder or other suitable device and form the melt into a film or mold it into solid objects. For example, containers (e.g., bottles in the case of polyethylene terephthalate resins) represent a very high volume use for pelletized polyester resins.